Interspecific coral competition does not affect the symbiosis of gall crabs (Decapoda: Cryptochiridae) and their scleractinian hosts

Abstract Coral reefs accommodate a myriad of species, many of which live in association with a host organism. Decapod crustaceans make up a large part of this associated fauna on coral reefs. Among these, cryptochirid crabs are obligately associated with scleractinian corals, in which they create dwellings where they permanently reside. These gall crabs show various levels of host specificity, with the majority of cryptochirids inhabiting a specific coral genus or species. Here, we report the first records of gall crabs living in association with two different Porites species in the Red Sea. Crescent‐shaped dwellings were observed in Porites rus and a Porites sp. in situ, and colonies with crabs were collected for further study in the laboratory. Using a combination of morphology and DNA barcoding, the crabs were identified as belonging to Opecarcinus, a genus only known to inhabit Agariciidae corals. The coral skeleton was bleached and studied under a stereo microscope, which revealed that the Porites corals overgrew adjoining agariciid Pavona colonies. We hypothesize that the gall crab originally settled on Pavona, its primary host of choice. Due to coral interspecific competition the Porites colony overgrew the adjacent Pavona colonies, resulting in a secondary and never before reported association of Opecarcinus with Porites. These findings suggest that cryptochirid crabs can adapt to the new microenvironment provided by a different coral host and survive competition for space on coral reefs.


| INTRODUC TI ON
Coral reefs are the most species-rich marine ecosystem. Due to their high habitat heterogeneity and the abundance of sessile host organisms inhabiting coral reefs, these ecosystems house the highest diversity of symbiotic relationships in the marine environment (Fisher et al., 2015;Hoegh-Guldberg, 1999). As major hermatypic reef builders, scleractinian corals are hosts to an extraordinary number of invertebrate species that rely on the corals for food, habitat, or refuge from predation (Castro, 1988;Paulay, 1997;Stella et al., 2011). In return, some symbionts may protect their hosts against corallivorous predators, increase coral health by clearing sediment from their hosts, or reduce their susceptibility to diseases (Montano et al., 2017;Pratchett, 2001;Rouzé et al., 2014;Stewart et al., 2006;Stier et al., 2010). Some of the most common symbionts of corals are brachyuran crabs, which often live in obligate commensal relationships with scleractinian corals (Castro, 2015). Despite exhibiting a secluded lifestyle, coral-dwelling gall crabs (Cryptochiridae Paulson, 1875) are a noteworthy, widely distributed and highly abundant group of reef-associated crustaceans (van Tienderen & van der Meij, 2016, and references therein). These diminutive crabs (<1 cm) are known to reside in dwellings in the coral skeleton induced by the crabs (Potts, 1915;van der Meij & Hoeksema, 2013). Their feeding biology and as a result also the nature of their symbiotic relationship has been debated (Abelson et al., 1991;Kropp, 1986;Potts, 1915;Simon-Blecher et al., 1999). A recent stable isotope study on three Atlantic cryptochirid species confirmed that these crabs mostly feed on coral tissue and/or mucus; hence, the obligate association to their scleractinian hosts involves a clear trophic relationship furthermore resulting in a strong host dependency .
Cryptochirids occur worldwide, mostly on tropical shallow-water reefs, but they have also been observed at depths below 500 m (Kropp & Manning, 1987). Currently, this family includes 53 species in 21 genera (WoRMS, 2023), yet several species complexes have recently been uncovered, and more species are awaiting description (Bähr et al., 2021;Xu et al., 2022). Gall crabs inhabit a broad range of corals from at least 66 genera (Castro, 2015). Their host specificity was used as a scheme to define cryptochirid genera in the past (Fize & Serène, 1957), but this has been overhauled by major taxonomic revisions of both corals and crabs (Fukami et al., 2008;Huang et al., 2014;Kropp, 1990;Terraneo et al., 2017). With the exception of the generalist species Troglocarcinus corallicola Verrill, 1908, all cryptochirid species are host specific; some at host species level, whereas other crab species associate with one or more closely related coral genera (e.g., Bähr et al., 2021;van der Meij, 2015;van der Meij et al., 2015;Xu et al., 2022).
It is noteworthy that the coral genera Acropora Oken, 1815 and Porites Link, 1807 are not among the more than 66 recorded coral host genera of gall crabs even though researchers (including the authors) have extensively examined them for the presence of cryptochirids (Fize & Serène, 1957;Kropp, 1988). Acropora and Porites are among the most abundant and diverse zooxanthellate coral genera on tropical reefs worldwide (Veron, 2000;Wallace, 1999) and arguably those with the most complex evolutionary and taxonomic history (Cowman et al., 2020;Terraneo et al., 2021). They also belong to the four coral genera with the highest symbiont fauna diversity (Stella et al., 2011: figure 5). Several brachyuran crab species inhabit Acropora corals (e.g., Tetralia spp. Dana, 1851), Domecia acanthophora (Desbonne in Desbonne & Schramm, 1867) and, to a lesser extent, also some predominantly branching Porites colonies (e.g., Mithraculus sculptus (Lamarck, 1818)) (Coen, 1988;Marin & Spridonov, 2007;van der Meij et al., 2022). There are few published records of brachyuran crabs in massive Porites. Coles (1982) reported on the domeciid crab The egg mass of the female crab and the fifth pereiopod of the male were removed for DNA extraction and molecular analysis based on the cytochrome oxidase subunit 1 (COI, partial) barcoding gene (Folmer et al., 1994

| RE SULTS
Crescent-shaped dwellings were observed in two different Porites colonies (Figure 1a,c). The corals were identified as Porites sp. (Figure 1a) and Po. rus (Figure 1c), based on the original species description and comparison with type material (Forskål, 1775) and scientific literature (Scheer & Pillai, 1983;Sheppard & Sheppard, 1991;Terraneo et al., 2021). The specimen identified as Porites sp. belongs to a species complex that contains, among others, Po. lutea Milne Edwards In the male crab, the anterior third of the carapace is white, whilst the posterior two-thirds of the carapace is dark rust. The eyes and AP are of similar hues as in the female crab. The merus of the third pereiopod is partially brown, whereas the fourth and fifth pereiopod have bright orange spots (Figure 1d). Based on their overall morphology and the crescent shape of their dwellings, the crabs were identified as belonging to the genus Opecarcinus Manning, 1987 (Kropp, 1990;Kropp & Manning, 1987). The COI sequences of both collected specimens were blasted against the dataset of Xu et al. (2022) & Serène, 1957). The species-rich genus Porites is, however, wellknown to harbor a diverse community of associated fauna, including mollusks, crustaceans, and polychaetes (Idris et al., 2022;Malay & Michonneau, 2014;Tsang et al., 2014;Zuschin et al., 2001). Decapod crabs are less commonly observed in association with encrusting or massive Porites corals as they seemingly prefer branching species for shelter (Marin & Spridonov, 2007;Patton, 1967;van der Meij et al., 2022). pits in the coral skeleton (Kropp, 1986  To date, there are no confirmed records of Opecarcinus in association with corals other than the Agariciidae Gray, 1847 (Kropp, 1989;van der Meij, 2014;Xu et al., 2022). After studying the in situ photographs in more detail, we noticed that there were Pa. varians corals adjacent to both of the Porites colonies (Figure 2a,b), one of the recorded hosts of Opecarcinus SET.11 in the Red Sea (Xu et al., 2022), in line with the observed host specificity in Cryptochiridae.
In both instances, the coral tissues of Porites and Pavona were directly touching (Figure 2a,b). The stereo microscope images of the bleached Po. rus fragments suggest that the adjacent Pa. varians colony was overgrown by Po. rus including the crab dwelling. Interspecific competition among scleractinian corals is a widespread phenomenon that shapes the ecology and bio-constructional potential of coral reefs (Connell, 1973;Connell et al., 2004;Horwitz et al., 2017;Karlson, 1999;Sheppard, 1979). Lang and Chornesky (1990) identified eight different competitive mechanisms, including overgrowth.
Porites rus seems to be particularly successful when competing with other Porites species (Idjadi & Karlson, 2007;Rinkevich & Sakai, 2001). Bleaching and the presence of pink color along the peripheral belt of the contacting tissue were specifically mentioned by Rinkevich and Sakai (2001), which is also visible in our Po. rus colony (Figure 2b). In both of the abovementioned studies, skeletal overgrowth was identified as the major driver of competition in their experiments. Such overgrowth is described as one of the main physical mechanisms adopted during inter-and intraspecific competition among scleractinian corals (Chadwick & Morrow, 2011). Based on these examples and our observations, we hypothesize that the Pavona colonies, including the gall crab dwellings, were likely overgrown by the adjacent Porites colonies, thus leading to a secondary association between Opecarcinus crabs and Porites corals.
How cryptochirids settle on their host coral is largely unknown, but their host specificity suggests fine-tuned (chemical) cues are at play. Corals have been observed to aggressively fight off settlement of nonassociated fauna (Liu et al., 2016); hence, successful settlement of Opecarcinus larvae on Porites corals seems an unlikely scenario. When considering the overgrowth of Pavona by Porites as likely, it is important to acknowledge that there could be a time constraint due to the life span of gall crabs and the growth rate of corals. Little is known about the maximum age cryptochirids can reach. Kotb and Hartnoll (2002)  on an abundant supply of potential recruits; however, the close correlation between hermit crab length and pit depth makes a good fit between a new recruit and available dwellings unlikely (Patton & Robertson, 1980). Edmondson (1933) included a section of the host of Cryptochirus coralliodytes Heller, 1860 [as C. rugusos], showing a 6-cm-deep pit in which, the crab resided, affirming the findings of Patton and Robertson (1980) that coral-dwelling decapods can be quite long-lived.
In order to judge whether the overgrowth of the Pavona colonies by the two Porites colonies within the life span of the gall crabs is a likely scenario, the growth rates of Porites corals should be taken into account. A study on Po. lutea in Indonesia revealed average growth rates between 1.11 and 1.20 cm/year (Zamani & Arman, 2016). Based on the analysis of coral cores Klein and Loya (1991) (Davies, 1989) over a time period of 2 years and reported on, respectively, a ninefold and sixfold increase. Lastly, in French Polynesia, Po. rus overgrew around 12% of the surface area of its inferior competitor P. lobata within a year (Idjadi & Karlson, 2007). Unfortunately, there are no data about the linear or radial extension rates for Po. rus in the Red Sea available, and thus the estimates made by these studies have to be interpreted with caution.
In case of Porites sp., we can confidently assume that the neighboring Pavona was overgrown within the lifetime of the gall crab since the dwelling is in close proximity (<1 cm) to the border between the two colonies ( Figure 2a). The crab dwelling on Po. rus is much further away from the bordering region between the colonies (Figure 2b). Based on the dwelling size, we estimate that it is approx. 4 cm away from the growth border of the colony. Nevertheless, given the observations about possible gall crab life span and the growth rates specified above, the distance of the dwelling to the colony edge seems plausible for an overgrowth scenario.
It is remarkable that the gall crabs were able to adapt to Porites as their secondary host, given the trophic relationship between gall crabs and corals , and because the carbohydrate composition of mucus secreted by corals differs between species (Wild et al., 2010). In addition, the surface mucus layer of corals is known to harbor a diverse and species specific bacterial community that is sensitive to changes in environmental variables (Brown & Bythell, 2005;Osman et al., 2020). While there is some uncertainty about the crabs diet, they rely at least in part on coral tissue/mucus as a source of nutrition Kropp, 1986;Simon-Blecher et al., 1999) and could thus be impacted by changes in mucus composition and associated microbial community. The Porites-inhabiting Opecarcinus crabs seem to have successfully adapted to the altered conditions and are either capable of feeding on Porites tissue/mucus or can switch to alternative feeding modes. Moreover, the female crab collected from Porites sp. was egg-bearing, hence still capable of obtaining the nutrients needed to invest energy in reproduction (Bähr et al., 2021).
Here, we reported on the first observation of Opecarcinus gall crab dwellings in Porites corals. Poritidae have never been reported as hosts for cryptochirid crabs, despite extensive search efforts by the authors and earlier scientists studying Cryptochiridae. The coral skeleton images revealed that Porites likely outcompeted their neighboring Pavona corals in the competition for space and unintendedly became "host" to the Pavona-associated Opecarcinus crabs.
Our findings suggest that Opecarcinus crabs are able to survive a substantial change in their obligate symbiotic relationship caused by interspecific coral aggression, and even thrive in their new surroundings with the female found capable of reproduction. Considering the trophic relationship between cryptochirids and their hosts, this feeding plasticity gives insights into the adaptive potential of gall crabs and hints at other processes (e.g., larval settlement) as drivers of host specificity.

ACK N OWLED G M ENTS
This research was undertaken in accordance with the policies and procedures of King Abdullah University of Science and Technology (KAUST). Permissions relevant for KAUST to undertake the research have been obtained from the applicable governmental agencies in the Kingdom of Saudi Arabia. We wish to thank KAUST Bioscience Core Lab and KAUST Coastal and Marine Resources Core Lab. We thank Alexander Kattan for his help in the field, and we are grateful for Erika Santoro's surprising find of a H. marsupialis s.l. dwelling in one of her experimental set-ups at the KAUST Coral Probiotic Village. Furthermore, we are grateful for the constructive comments provided by the reviewers.

FU N D I N G I N FO R M ATI O N
This project was supported by funding from KAUST baseline research funds of F. Benzoni.

CO N FLI C T O F I NTER E S T S TATEM ENT
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Sequences are deposited to GenBank under accession numbers OQ430764 and OQ430765.